Agressive behavior

I have a wonderful 12 yr old Jack Russel, his companion of 10 yrs died

in Jan 2009, in june of 2009 I purchased a JRT pup. The 2 got along
quite well. Now it is Dec 2009 & my older JRT is getting very aggressive
with the younger dog. The older JRT is picking fights over everything.
Any suggestions on what to do with the older JRT. Note when they fight
I do break it up with a squirt bottle & roll the older JRT onto his back.